Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...

Hi Valery, yes, this is something i've done. It does work, in theory, but the problem I had that i could never really deal with repeatably was that of reflections between the 2 etalons. The obvious answer is to tilt the etalons to shift the reflection, but, as you know, doing this blue shifts the etalons response, so this isn't a solution. I worked with this for a while, looking at things like using a polarizing filter between the 2 etalons to kill the reflection with limited success. In the end the project kinda dwindled away. Looking I have some results on my website http://brierleyhillsolar.blogspot.co.uk ... m-3rd.html link to big version here https://farm8.staticflickr.com/7303/943 ... 590c_o.jpg

Valery,
I've used a PST etalon double stack...the "fiddly" tuning (and rotation) of the etalons was more hassle than I was prepared to accept.
I do have a special PST/PST adaptor - drawing available
The PST etalon seems to work well double stacked with an upfront SM60